Optical amplification apparatus

ABSTRACT

An optical amplification apparatus for adjusting optical power of wavelength-multiplexed signal light at respective wavelengths and for adjusting the optical output power at the respective wavelengths and a deviation of the optical output power between the wavelengths. An optical power adjusting unit for receiving inputted light having signal light at a plurality of different wavelengths (λ 1 , λ 2 , λ 3 , . . . , λ N ) multiplexed thereon, amplifying or attenuating light at least one wavelength included in the received light independently of light at different wavelengths from the wavelength of the light, is provided before or after an optical amplifier unit for amplifying the light having the signal light at the plurality of different wavelengths multiplexed thereon. Further, a control unit is provided for controlling the gain of amplification or attenuation performed by the optical power adjusting unit and the gain of amplification performed by the optical amplifier unit, respectively.

This application is a 37 CFR §1.60 continuation of prior applicationSer. No. 08/626,951, filed Apr. 3, 1996 (allowed) now U.S. Pat. No.5,675,432.

BACKGROUND OF THE INVENTION

The present invention relates to optical amplification apparatus for usein optical transmission systems and so on.

As a reduction in cost has been demanded for optical transmissionsystems, a wavelength-multiplex optical transmission has been taken intoconsideration for transmitting signal light at one or more mutuallydifferent wavelengths on a single transmission fiber. It is also thoughtthat an amplifier suitable for use in such wavelength-multiplex opticaltransmission is an optical amplification apparatus which has a wideamplification wave band and is capable of achieving the amplificationwith less noise.

It is known, however, that a rare metal added optical fiber and asemiconductor optical amplifier constituting the above-mentioned opticalamplification apparatus have a gain dependency so that optical outputsand gains at respective wavelengths present deviations due to thedifference in wavelengths after amplification. For this reason, theoptical power at different wavelengths after transmission involves adeviation due to the difference in wavelengths. Particularly, if anumber of optical amplifiers are used to relay signal light at multiplestages, the deviation of optical power between different wavelengths,generated at respective relay stages, are accumulated as the signallight is relayed from one stage to next, thus increasing the deviationof optical power between the different wavelengths.

In the wavelength-multiplex optical transmission, since the wavelengthsignal having the lowest power of all multiplexed wavelengths must beregarded as a lower limit value of received power after transmission, amaximum transmission distance in the wavelength-multiplex transmissionis limited by the wavelength signal having the lowest power. Thus, it isof great importance to reduce the deviation of power between differentwavelengths in the output of an optical amplification apparatus, inorder to extend a maximum relay transmission distance.

To solve this problem, an article titled "Collective Smoothing ofMultiple Wavelength Amplification Characteristics of Fiber OpticAmplifier Using Fiber Amplification Ratio Control" Technical Reports ofthe Institute of Electronics, Information and Communications OCS94-66,OPE94-88 (1944-11) has proposed the following technique.

FIG. 1 illustrates the configuration of an optical amplificationapparatus according to the technique disclosed in the article. Referringspecifically to FIG. 1, the optical amplification apparatus includes anerbium-added optical fiber 51, an optical isolator 52, a light combiner53, an excitation light source 54, an optical attenuator 55, an opticalcoupler 56 for splitting the output of the optical attenuator 55, and alight detector 57 for detecting split light.

In the disclosed technique, the illustrated optical amplificationapparatus is controlled by an auto fiber gain controller (AFGC) suchthat a fiber gain remains at 12 dB, thereby minimizing a deviation ofgain between respective wavelengths. In addition, an auto powercontroller (APC) implemented by the optical attenuator 55 is used toprevent a change in relay amplification ratio from affecting the gainspectrum.

It has been reported that, according to theoretical calculations, theoptical amplification apparatus presented a minimum gain deviationbetween respective wavelengths, which is 0.12 dB or less, when theerbium-added optical fiber 50 had a length of 11 meters, assuming that adeviation of gain between the respective wavelengths of input light was0 dB. It has been also reported that after the optical amplificationapparatus has been used to relay light having four different wavelengthsmultiplexed thereon 60 times, a gain deviation was 1.5 dB or less.

Optical losses during transmission may vary from one case to another dueto a difference in fiber loss within each relayed area, a difference inoptical power between adjacent wavelengths, and so on. Additionally, inan actual use, relayed distances and fiber losses in respective areasare not always constant. It is therefore difficult to predict adeviation of gain between respective wavelengths and optical power atthe respective wavelengths in an actual use. Therefore, the opticalamplification apparatus illustrated in FIG. 1 has a problem in an actualuse that if an input level changes or if a deviation of gain occursbetween input wavelengths, the optical amplification apparatus cannotreduce a deviation of gain between output wavelengths to 0 dB.

Also, when the optical amplification apparatus illustrated in FIG. 1 isused, if an independent fluctuation in output power of signal light at acertain wavelength caused by an external factor, for example, is to besuppressed, stable output power of signal light at the remainingwavelengths is also suppressed simultaneously, thus adversely affectingthe stability of the output power of the signal light at the differentwavelengths.

Further, since the optical amplification apparatus illustrated in FIG. 1is dependent on the gain thereof for establishing an optimal conditionfor eliminating the deviation of gain between wavelengths, it cannotfreely set outputs of signal lights. More specifically, since a relayeddistance is limited by the optical amplification apparatus the freedomin designing the architecture of a transmission system is restricted.The optical amplification apparatus illustrated in FIG. 1 additionallyhas a problem that it must be optimized to eliminate a deviation of gainbetween wavelengths in each relay area.

SUMMARY OF THE INVENTION

It is a principal object of the present invention to provide an opticalamplification apparatus which is capable of arbitrarily adjustingoptical output power at respective wavelengths of wavelength-multiplexedsignal light and a deviation in optical power between the respectivewavelengths.

It is another object of the present invention to provide an opticalamplification apparatus which uniformly increases or decreases inputpower of signal light at respective wavelengths inputted thereto and anamplification ratio of the optical amplification apparatus to therebygenerate an output which is dependent on an increase or a decrease ofthe input power.

To achieve the above objects, the present invention positions an opticalpower adjusting means which receives inputted light having signal lightat a plurality of different wavelengths multiplexed thereon foramplifying or attenuating light at at least one wavelength included inthe received light independently of the remaining light at differentwavelengths before or after an optical amplifying means for amplifyingthe light having the signal light at the plurality of differentwavelengths multiplexed thereon. Further, a control means is providedfor controlling the gain of amplification or attenuation performed bythe optical power adjusting means and the gain of amplificationperformed by the optical amplifier, respectively.

The present invention will be explained below in connection with anexample in which the optical power adjusting means is provided before orafter the optical amplifying means.

A rare earth added optical fiber or a semiconductor amplifier commonlyused as the optical amplifying means has output power dependent on inputpower on condition that excitation power is constant. This also applieswhen multiplexed light having light at wavelengths λ₁, λ₂, λ₃, . . . ,λ_(N) multiplexed thereon is simultaneously amplified. Therefore, if theoptical amplifying means is driven to increase or decrease the inputpower of the light at the respective wavelengths, it is possible toproduce output power dependent on the increased or decreased inputpower.

Thus, in the present invention, the optical power adjusting means ispositioned before the optical amplifying means. The optical poweradjusting means receives light having multiplexed thereon light at aplurality of wavelengths and amplifies or attenuates light at at leastone wavelength included in the received light independently of otherlight at different wavelengths from the wavelength of the light to beamplified or attenuated. The optical power adjusting means adjusts adeviation of optical power between the wavelengths of the light at therespective wavelengths inputted to the optical amplifying means, andthereafter the optical amplifying means simultaneously amplifies thelight having the light at the respective wavelengths multiplexedthereon, thereby adjusting the power of the light at the respectivewavelengths and the deviation of optical power between the wavelengthsto desired values.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described inconjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram illustrating the configuration of aconventional optical amplification apparatus;

FIG. 2 is a block diagram illustrating an example of the configurationof an optical transmission system according to a first embodiment of thepresent invention;

FIG. 3 is a block diagram illustrating the configuration of an opticalamplification apparatus according to the first embodiment of the presentinvention;

FIG. 4 is a block diagram illustrating a first exemplary configurationof an optical power adjusting unit according to the first embodiment ofthe present invention;

FIG. 5 is a block diagram illustrating the configuration of a lightcombiner/separator according to the first embodiment of the presentinvention;

FIG. 6 is a block diagram illustrating the configuration of an opticalgain adjuster according to the first embodiment of the presentinvention;

FIG. 7 is a block diagram illustrating a control unit according to thefirst embodiment of the present invention;

FIG. 8 is a block diagram illustrating a second exemplary configurationof the optical power adjusting unit according to the first embodiment ofthe present invention;

FIG. 9 is a block diagram illustrating a third exemplary configurationof the optical power adjusting unit according to the first embodiment ofthe present invention;

FIG. 10 is a block diagram illustrating a general configuration of anoptical amplification apparatus according to a second embodiment of thepresent invention;

FIG. 11 is a block diagram illustrating a first exemplary configurationof the optical amplification apparatus according to the secondembodiment;

FIG. 12 is a block diagram illustrating the configuration of a controlunit in the first exemplary configuration of the optical amplificationapparatus according to the second embodiment of the present invention;

FIG. 13 is a block diagram illustrating a second exemplary configurationof the optical amplification apparatus according to the secondembodiment;

FIG. 14 is a block diagram illustrating a third exemplary configurationof the optical amplification apparatus according to the secondembodiment;

FIG. 15 is a block diagram illustrating a control unit in a firstexemplary configuration of an optical amplification apparatus accordingto a third embodiment of the present invention;

FIG. 16 is a block diagram illustrating the configuration of the opticalamplification apparatus according to the third embodiment of the presentinvention;

FIG. 17 is a block diagram illustrating in detail the configuration ofthe optical amplification apparatus according to the third embodiment ofthe present invention; and

FIG. 18 is a block diagram illustrating the configuration of an opticalamplification apparatus according to a fourth embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Optical amplification apparatuses according to the present inventionwill hereinafter be described in connection with preferred embodimentsthereof.

First, a first embodiment will be described with reference to FIG. 2illustrating the configuration of an optical transmission systemaccording to the first embodiment.

The optical transmission system of FIG. 2 comprises an opticaltransmitter 1 for emitting signal light; an optical booster amplifier 2for amplifying the power of the emitted signal light; a transmissionfiber 3 for transmitting the signal light; an optical relay 4 foramplifying and relaying the signal light; an optical pre-amplifier 5 foramplifying the transmitted signal light; and an optical receiver 6 forreceiving the signal light amplified by the optical pre-amplifier 5.

The optical amplification apparatus according to the first embodimentmay be used as the optical booster amplifier 2, the optical relay 4, andthe optical pre-amplifier 5 in the optical transmission systemillustrated in FIG. 1.

The first embodiment will be described in connection with the opticalbooster amplifier 2, as representative, implemented by the opticalamplifier according to the first embodiment.

FIG. 3 illustrates the configuration of the optical booster amplifier 2.In the first embodiment, the optical booster amplifier 2 is suppliedwith input signal light Pin having light at three different wavelengthsmultiplexed thereon from the optical transmitter 1. The threewavelengths are λ₁ =1547 nm (nanometers, =10⁻⁹ meter), λ₂ =1552 nm andλ₃ =1557 nm in this particular example. As illustrated in FIG. 3, theoptical booster amplifier 2 is composed of an optical isolator 7, anoptical power adjusting unit 8, an optical amplifier unit 9, an opticalisolator 13, and a control unit 14. The optical amplifier unit 9 iscomposed of a rare earth added optical fiber 10, an excitation lightsource 11, and a light combiner 12. Used in the first embodiment as therare earth added optical fiber 10 is an erbium-added optical fiber.Also, as the excitation light source 11, a semiconductor laser having awavelength at 1480 nm is used in the first embodiment.

In the optical booster amplifier 2, the input signal light Pin issupplied to the optical power adjusting unit 8 through the opticalisolator 7. The optical power adjusting unit 8 adjusts the optical powerof light at respective wavelengths (λ₁, λ₂, λ₃, . . . , λ_(N)) as wellas a deviation of the optical power between the respective light at therespective wavelengths. Then, the adjusted signal light Pin is outputtedto the optical amplifier unit 9.

In the optical amplifier unit 9, excitation light emitted from theexcitation light source 11 flows into the erbium-added optical fiber 10to excite the erbium-added optical fiber 10. Thus, the signal lightinputted from the optical power adjusting unit 8 into the erbium-addedoptical fiber 10 is amplified and outputted to the optical isolator 13through the light combiner 12. Then, an output signal light Pout havinglight at wavelengths λ₁, λ₂, λ₃ multiplexed thereon is outputted fromthe optical isolator 13. Here, the excitation light source 11 iscontrolled in terms of an excitation light amount by the control unit14.

It should be noted that the excitation light in the optical amplifierunit 9 may be inputted from a stage before the erbium-added opticalfiber 10. In addition, a semiconductor amplifier may be used as theoptical amplifier unit 9.

The control unit 14 controls the optical amplifier unit 9 and theoptical power adjusting unit 8 to adjust the optical output of theoptical amplifier unit 9 and a deviation of optical power between themultiplexed wavelengths for each wavelength, as will be described belowin detail.

Next, the optical power adjusting unit 8 will be described withreference to FIG. 4 which illustrates the internal configuration of theoptical power adjusting unit 8. As illustrated, the optical poweradjusting unit 8 comprises light combiner/separator units 15, 16 eachfor combining signal light at wavelengths λ₁, λ₂, λ₃ and separatingcombined light into light at the wavelengths λ₁, λ₂, λ₃, and opticalgain adjusters 17b, 17c for adjusting the optical power of the light atwavelengths λ₂, λ₃. The signal light at wavelength λ₁ is not adjusted.The light combiner/separator units 15, 16 have a structure including acombination of two 1×3 optical star couplers 18, 19 and optical filters20a, 20b, 20c. The optical gain adjusters 17b, 17c are composed of rareearth added optical fibers 21b, 21c; excitation light sources 22b, 22c;and light combiners 23b, 23c, respectively. In the first embodiment,light emitting diodes at a wavelength of 820 nm are used as theexcitation light sources 22b, 22c and erbium-added optical fibers as therare earth added optical fibers 21b, 21c.

The deviation of optical power between wavelengths can be relatively setbased on light at one wavelength. For example, if the gain of theoptical amplifier unit 9 is determined by setting an excitation lightamount of the excitation light source 11 such that the optical outputpower of light at wavelength is +10 dBm, and excitation light amounts ofthe excitation light sources 22b, 22c in the optical gain adjusters 17b,17c are adjusted to determine the gains of light at wavelengths λ₂, λ₃in accordance with the setting of the optical amplifier unit 9, theoutput power of the light at the respective wavelengths and thedeviation of optical power between the wavelengths can be arbitrarilyadjusted. The control for adjusting the excitation light amountsoutputted by the excitation light sources 11, 22b, 22c is performed bythe control unit 14.

While FIG. 4 illustrates a configuration of the optical power adjustingunit 8 without optical gain adjuster for the light at wavelength λ₁,alternatively, the optical power adjusting unit 8 may be provided withan optical gain adjuster for the light at wavelength λ₁ and not providedwith an optical gain adjuster for the light at wavelength λ₂ or λ₃.Also, according to the characteristic of the erbium-added optical fiber10 used in the first embodiment, there is few deviation of gain betweenlight at 15544 nm and light at 1565 nm. Thus, when wavelengths of 1544nm and 1565 nm are used as part of wavelengths to be multiplexed onsignal light, the gain may be adjusted for the rest of wavelengths bythe optical gain adjuster 17 without combination and separation of thewavelengths of 1544 nm and 1565 nm and without adjusting the gains atthe wavelengths of 1544 nm and 1565 by the optical gain adjuster 17. Inthis way, the configuration of the optical power adjusting unit 8 can besimplified.

Next, the operation of the light combiner/separator units 15, 16 in theoptical power adjusting unit 8 will be explained with reference to FIG.5. It should be noted that the illustration of the optical gainadjusters 17b, 17c are omitted in FIG. 5 for clarifying the explanation.

Referring specifically to FIG. 5, signal light Pin inputted to the lightcombiner/separator units 15, 16 is equally split into three which aresupplied to three optical filters 20a-20c. The optical filters 20a-20coutput light at wavelengths λ₁, λ₂, λ₃, respectively. Specifically, theoptical filter 20a having a pass band of 1547 nm±1 nm passes only lightat wavelength λ₁ (=1547 nm); the optical filter 20b having a pass bandof 1552 nm±1 nm passes only light at wavelength λ₂ (=1552 nm); and theoptical filter 20c having a pass band of 1557 nm±1 nm passes only lightat wavelength λ₃ (=1557 nm).

Among the light, the light at wavelengths λ₂, λ₃ is adjusted the gainsby the optical gain adjusters 17b, 17c, respectively, and combined withthe light at wavelength λ₁ outputted from the optical filter 20a by a1×3 optical star coupler 19.

The optical filter 20a may be implemented by any low pass filter, forexample, one having a wavelength pass band of approximately 1548 nm orless, as long as it blocks light at wavelengths λ₂, λ₃. Likewise, theoptical filter 20c may be implemented by any high pass filter, forexample, one having a wavelength pass band of approximately 1556 nm ormore, as long as it blocks light at wavelengths λ₁, λ₃.

In the operation mentioned above, the light at three wavelengthsrespectively suffers from substantially a uniform loss of -5 dB due tothe 1×3 optical star coupler 18, and from substantially a uniform lossof -1 dB due to the respective optical filters 20a, 20b, 20c. Further,the light at three wavelengths also suffers from substantially a uniformloss of -5 dB due to the 1×3 optical star coupler 19 at the final stage.Thus, the light at the three wavelengths uniformly undergoes an opticalloss amounting to -11 dB. This means that the optical gain adjuster 17is the only factor inside the optical power adjusting unit 8 whichcauses a deviation of gain between wavelengths and the remaining opticalparts do not contribute to the deviation of gain between wavelengths. Itwill be understood that if the number of wavelengths of lightmultiplexed on signal light is increased as λ₁, λ₂, λ₃, λ₄, . . . ,λ_(N), the number of branches provided by the star coupler 18, thenumber of the optical filters 20, and the number of the optical gainadjusters 17 may be correspondingly increased.

Next, the operation of the optical gain adjuster 17 will be explained indetail with reference to FIG. 6.

Since the same components are used for the optical gain adjusters 17b,17c in the first embodiment, the optical gain adjuster 17b is onlyexplained below.

Excitation light from the excitation light source 22b in FIG. 6 flowsinto the erbium-added optical fiber 21b at its rear end through thelight combiner 23b and excites the erbium-added optical fiber 21b. Lightat wavelength λ₂ is inputted from the front end of the erbium-addedoptical fiber 21b, subjected to amplification or attenuation in theerbium-added optical fiber 21b, and outputted therefrom. The excitationlight source 22b is controlled by the control unit 14. The excitationlight may be inputted from a stage before the erbium-added optical fiber21b. Also, the optical gain adjuster 17b may be implemented by asemiconductor amplifier, in which case an excitation current iscontrolled by the control unit 14.

Generally, an optical amplification medium used for an optical amplifieracts as a medium for amplifying light when excitation power is flowingthereinto, whereas it acts as a medium for attenuating light if a smallamount of excitation power or no excitation power is flowing thereinto.Since the optical gain adjuster 17 according to the first embodiment iscomposed of the rare earth added optical fiber 21, the excitation lightsource 22, and the light combiner 23, the optical gain adjuster 17functions to have a negative gain when a less amount of excitation poweris supplied thereto and functions to have a positive gain when a largeramount of excitation power is supplied thereto to enable amplificationof light.

Since conventional optical attenuators require external control using amotor or the like, they are disadvantageous in large size and slowcontrol speed. In contract, the optical gain adjuster 17 of the firstembodiment only relies on increase and decrease of the excitation powerto adjust optical output power, so that the adjustment of the gain oflight, including an attenuation direction, can be simply andinstantaneously carried out. Thus, no motor is required for the control.Further, since the amplification wavelength band has a sufficient extentto fully cover the band of multiplexed signal light, the same opticalgain adjusters 17 can be used for light at respective wavelengths.

In the first embodiment, the optical gain adjuster 17b is formed of theerbium-added optical fiber 21b which is made of the same material as theerbium-added optical fiber 10 of the optical amplifier unit 9 at asubsequent stage by the following reason.

Specifically, a control speed (approximately 1-5 ms) provided by anexcitation light amount from the excitation light source 11 in theoptical amplifier unit 9, controlled by the control unit 14 as describedabove, is substantially determined by a relaxation lifetime when theerbium-added optical fiber 10 of the optical amplifier unit 9 is excitedby the excitation light. Likewise, an adjusting speed of the opticalgain adjuster 17b is determined by a relaxation lifetime when theerbium-added optical fiber 21a is excited by the excitation light, sothat the adjusting speed is substantially the same as the control speedof the optical amplifier unit 9. Thus, the adjustment can be carried outat a high speed equivalent to the control speed provided by theexcitation light amount of the excitation light source 11 in the opticalamplifier unit 9 controlled by the control unit 14. Moreover, theoptical gain adjuster 17b does not perform excessively fast adjustmentwhich would adversely affect signal modulated waveforms. In addition,the wavelength amplification band of the erbium-added optical fiber 10in the optical amplifier unit 9 can be made completely the same as thewavelength adjusting band of the optical gain adjuster 17b.

For the reason mentioned above, the present inventors think that therare earth added optical fiber 21b in the optical gain adjuster 17b andthe rare earth added optical fiber 10 in the optical amplifier unit 9 ata subsequent stage are preferably made of the same material. However,any other rare earth added optical fibers may be used instead of theerbium-added optical fiber 21b.

The erbium-added optical fiber 21b may have a length of approximately 3m because an excessive amplification characteristic is not necessary, sothat approximately 1/10 is only required for the erbium-added opticalfiber 21b as compared with the length of the erbium-added optical fiber10 used for the optical amplifier unit 9.

The output of the light emitting diode 22b at 830 nm used for theexcitation light source 22b may be 20 mW or less. Generally, while highpower semiconductor lasers having a wavelength band centered at 980 nmor 1480 nm providing a high gain efficiency are effective when a rareearth added optical fiber is used as an amplifying medium, a lightsource having a wavelength band of a low gain efficiency or a low powerlight source is sufficiently applicable to the excitation light source22b used for the optical gain adjuster 17b. Thus, an applicable range ofthe excitation light source 22b usable for the optical gain adjuster 17bis quite wide. For example, lower power light sources having awavelength band near 520 nm, near 660 nm, near 820 nm, near 980 nm, andnear 1480 nm may be used for this purpose. Particularly, a lightemitting diode having a wavelength band near 830 nm is available at alow price, so that the use of this light emitting diode permits theoptical gain adjuster 17b according to the first embodiment to beimplemented at a lower cost.

Next, the control unit 14 will be described with reference to FIG. 7.The controller 14 controls an excitation light amount of the excitationlight source 11 in the optical amplifier unit 9, and excitation lightamounts of the excitation light sources 22b, 22c in the respectiveoptical gain adjusters 17b, 17c in the optical power adjusting unit 8,in order to adjust the optical output and a deviation of power betweendifferent wavelengths for each wavelength.

FIG. 7 illustrates the internal configuration of the control unit 14. Amemory unit 24 previously has stored several control parameters 25. Forexample, a plurality of sets of an excitation light amount of 1480 nmsemiconductor laser functioning as the excitation light source 11 in theoptical amplifier unit 9 and excitation light amounts of 820 nm lightemitting diodes functioning as the excitation light sources 22b, 22cinternal to the respective optical gain adjusters 17b, 17c are stored inthe memory unit 24 as the parameters 25 such that one of these sets isselected by a controller 26 in response to input information from theoutside.

The controller 26 controls an excitation light amount of the 1480 nmsemiconductor laser serving as the excitation light source 11 in theoptical amplifier unit 9 and excitation light amounts of the 820 nmlight emitting diodes functioning as the excitation light sources 22b,22c internal to the respective optical gain adjusters 17b, 17c inresponse to a selected one of the parameters 25. The parameters 25 areprovided for each set of powers at respective wavelengths of inputlight, so as to be selected by the controller 26 in response to actualinput optical power instructed thereto from the outside.

Table 1 shows the control parameters 25 provided for each set of powersat respective wavelengths of input light. According to the controlparameters 25, the optical output power can be set to +10 dBm at eachwavelength for any combination of power of input light at the respectivewavelengths, so that the deviation of output power between therespective wavelengths can be set to 0 dB. In the first embodiment,input power of the optical transmitter 1 provided to the optical boosteramplifier 2 is accurately controlled to -2 dBm at either of wavelengthsλ₁, λ₂, λ₃ and substantially no fluctuations in input power and nodeviation of output power between inputted signal light at therespective wavelengths are found, so that the controller 26 selects theparameters 25 of a shaded column in Table 1.

                                      TABLE 1                                     __________________________________________________________________________     ##STR1##                                                                     __________________________________________________________________________

Alternatively, the control parameters 25 may describe the values ofcurrents to be supplied to the excitation light sources 11 and 22. Alsoalternatively, the control parameters 25 may be previously stored foreach combination of power of output light such that the controller 26selects one in response to a desired combination of power of outputlight instructed from the outside.

Table 2 shows an example of the control parameters 25 which are set foreach combination of power of output light. The use of the controlparameters 25 shown in Table 2 provides output light power having anarbitrary deviation between outputs at different wavelengths. The inputpower of signal light at λ₁, λ₂, λ₃ to the optical amplificationapparatus of the optical transmitter 1 in the first embodiment isaccurately controlled to -2 dBm so that substantially no fluctuations ininput power and no deviation of input power between input light atdifferent wavelengths are found. For maintaining the optical power ofsignal light at respective wavelengths uniformly to -25 dBm after beingtransmitted a distance of 120 km under such a condition, the controller26 may select, for example, parameters 25a indicated by shading in Table2 so as to cancel a deviation of power between the respectivewavelengths caused by the attenuation of power during the transmission.The optical output power at the respective wavelengths may be determinedin accordance with a transmission distance and losses possibly caused bythe transmission fiber 3 so as to provide an optimal condition.

                                      TABLE 2                                     __________________________________________________________________________     ##STR2##                                                                     __________________________________________________________________________

Also, in this case, the control parameters 25 may represent values ofcurrents to be supplied to the excitation light sources 11 and 22.Further alternatively, the control parameters 25 may be set so as tocontrol converted gains instead of the output power.

Alternatively, the optical power adjusting unit 8 may be configured asillustrated in FIG. 8. The configuration illustrated in FIG. 8 differsfrom that illustrated in FIG. 4 in that the former comprises opticalgain adjusters 17a, 17b, 17c on paths for all of signal light at λ₁, λ₂,λ₃, respectively. The configuration of FIG. 8 enables optical power atthe respective wavelengths to be adjusted completely independent of eachother, so that the adjustment accuracy is also improved. In this case,the control parameters 25 stored in the memory unit 14 should includeone describing the power of excitation light for wavelength λ₁ from theexcitation light source 22a in the optical gain adjuster 17a. It shouldbe noted that the configuration of FIG. 8 can also support an increasednumber of multiplexed signal light at different wavelengths such as λ₁,λ₂, λ₃, λ₄, . . . , λ_(N). Further, in the configuration of FIG. 8, theoptical gain adjusters 17a, 17b, 17c may be implemented by semiconductoramplifiers.

Further alternatively, the optical power adjusting unit 8 may beconfigured as illustrated in FIG. 9. Specifically, the optical poweradjusting unit 8 illustrated in FIG. 9 comprises an optical isolator 27,a rare earth added optical fiber 28, an excitation light source 29controlled by the control unit 14, a light combiner 30, light sources31a, 31b, 31c externally controlled by the control unit 14 to generatelight at one or more different wavelengths λ₁, λ₂, λ₃, a 1×4 opticalstar coupler 32 for combining light from the light sources 31a, 31b, 31cin the direction opposite to the flow of signal light.

The light generated from the light sources 31a, 31b, 31c is in the samewavelength band as that of the multiplexed signal light. With thisconfiguration, if optical power of signal light at a certain wavelengthis larger than other signal light at different wavelengths, the opticaloutput of the light source 31 emitting light in the same wavelength bandas the signal light is increased to consume amplification energy withinthe rare earth added optical fiber 28, whereby the amplification ratioof the signal light at the wavelength can be reduced. Conversely, ifoptical power of an signal light at a certain wavelength is smaller thanothers, the optical output of the light source 31 emitting light in thesame wavelength band as this signal light is decreased to keepamplification energy within the rare earth added optical fiber 28,whereby the amplification ratio of the signal light at the wavelengthcan be increased.

In the configuration illustrated in FIG. 9, the light from the lightsource 31 is incident in the direction opposite to the flow of thesignal light in order to prevent the light from the light source 31 frommixing with the signal light. Also, the optical isolator 27 is arrangedbefore the rare earth added optical fiber 28 to block the light from thelight source 31 amplified in the rare earth added optical fiber 28 anddirecting in the opposite direction, thus preventing parts arrangedbefore the optical power adjusting unit 8 from being affected by thelight from the light source 31. The configuration illustrated in FIG. 9also uses an erbium-added optical fiber for the rare earth added opticalfiber 28 and a 830 nm semiconductor laser for the excitation lightsource 29. Then, the controller 26 selects a set of control parameters25 as shown in Table 3 in order to set optical outputs at +10 dBm forrespective wavelengths and a deviation of output power betweenwavelengths at 0 dB.

                  TABLE 3                                                         ______________________________________                                        Input Power            (-2, -2, -2)                                           (.sub.1, .sub.2, .sub.3) [dBm]                                                Output Power           (+10, +10, +10)                                        (.sub.1, .sub.2, .sub.3) [dBm]                                                Excitation Light Source [mW] (11)                                                                    95 mW                                                  Optical      Excitation Light                                                                            30 mW                                              Power        Source [mW] (29)                                                 Adjusters    Light Source .sub.1 (31a)                                                                    0 mW                                              [dB]         Light Source .sub.2 (31b)                                                                    5 mW                                                           Light Source .sub.3 (31c)                                                                    7 mW                                              ______________________________________                                    

It should be noted that also in this configuration, the controlparameters 25 may be the values of currents applied to the excitationlight sources 11, 22 and to the light sources 31a, 31b, 31c. Further,portions of the light combiner 30, the erbium-added optical fiber 28 andthe excitation light source 29 may be replaced with semiconductoramplifiers.

Next, a second embodiment of the optical amplification apparatusaccording to the present invention will be described below. The opticalamplification apparatus according to the second embodiment may also beused as the optical booster amplifier 2, the optical relay 4, and theoptical pre-amplifier 5 in the optical transmission system illustratedin FIG. 2.

FIG. 10 illustrates the configuration of the optical amplificationapparatus according to the second embodiment. The configuration of theoptical amplification apparatus according to the second embodimentdiffers from the configuration of the optical amplification apparatusaccording to the first embodiment (see FIG. 3) in that the formerfurther comprises a light splitter 33 for splitting part of an opticaloutput; an output monitor unit 34 for detecting power of light atwavelengths λ₁, λ₂, λ₃ included in light split by the light splitter 33or a deviation of power between these wavelengths; a light splitter 35for splitting part of inputted light; and an input monitor unit 36 fordetecting power of light at wavelengths λ₁, λ₂, λ₃ included in lightsplit by the light splitter 35 or a deviation of power between thesewavelengths. Also, in the second embodiment, the control unit 14automatically controls respective optical gain adjusters 17 in theoptical power adjusting unit 8 and the excitation light source 11 in theoptical amplifier unit 9 in response to power of light at the respectivewavelengths or a deviation of power between the wavelengths detected atleast one of the output monitor unit 34 and the input monitor unit 36such that the power of the light at the respective wavelengths includedin output light or the deviation of power between the wavelengthsreaches a predetermined value.

According to the configuration described above, even if input power tothe optical amplification apparatus, output power and/or a conversiongain exhibit fluctuations, the optical output power at respectivewavelengths and the deviation of power between the wavelengths can beautomatically controlled to respective predetermined values. Also, evenif light at any wavelength only fluctuates, the output power of thefluctuating light at the wavelength can be controlled to a predeterminedvalue without affecting the output power of light at the remainingwavelengths. Further, the excitation light source 11 in the opticalamplifier unit 9 or the like suffers from aging changes, the opticaloutput power at the respective wavelengths and the deviation of powerbetween the wavelengths can be always maintained at a predeterminedvalue, thus making it possible to improve the stability and reliabilityof the entire optical amplifier.

It should be noted that among the light splitter 33, the output monitorunit 34, the light splitter 35 and the input monitor unit 36 newlyproposed in the second embodiment, either a combination of the lightsplitter 33 and the output monitor unit 34 or a combination of the lightsplitter 35 and the input monitor unit 36 may only be provided for theoperation mentioned above.

In the following, the second embodiment will be described for the casewhere a combination of the light splitter 33 and the output monitor unit34 are only chosen from among the light splitter 33, the output monitorunit 34, the light splitter 35 and the input monitor unit 36 newlyproposed in the second embodiment.

FIG. 11 illustrates in greater detail the optical amplificationapparatus of the case mentioned above.

Referring specifically to FIG. 11, the output monitor unit 34 iscomposed of a 1×3 optical star coupler 37; optical couplers 38a, 38b,38c; optical filters 20a, 20b, 20c; optical couplers 39a, 39b, 39c; andoptical detectors 40a, 40b, 40c. The optical filters 20a, 20b, 20c inFIG. 11 are identical to the optical filters 20a, 20b, 20c illustratedin FIG. 4. In other words, the optical filters 20a, 20b, 20c are alsoused as part of the output monitor unit 34. The remaining parts in FIG.11 are identical to those denoted by the same reference numerals in FIG.4.

In the configuration mentioned above, monitor light split from part ofan output by the light splitter 33 formed of an optical coupler is againsplit by the 1×3 star coupler 37, and enters through the opticalcouplers 38a, 38b, 38c in the direction opposite to the flow of signallight within the optical power adjusting unit 8. The entering light ispassed through associated optical filters 20a, 20b, 20c corresponding torespective wavelengths to extract light at the respective wavelengths.Then, the extracted light on each path is partially split by theassociated optical coupler 39a, 39b or 39c. The power of the split lightat the respective wavelengths is detected by the optical detectors 40a,40b, 40c, respectively, and notified to the control unit 14.

As will be understood from the foregoing, the optical filters 20a, 20b,20c in the configuration illustrated in FIG. 11 perform a function ofextracting necessary wavelength components from monitor light and afunction of separating input light into respective wavelengthcomponents. The latter function has also been carried out in the firstembodiment. Since no additional optical filters are required for theoutput monitor unit 34, the configuration can be simplified.

Also, in the configuration illustrated in FIG. 11, since the monitorlight split from output light is inputted to the optical fibersconnected to the optical filters 20a, 20b, 20c before the optical gainadjusters 17a, 17b, 17c, the monitoring light will not be affected bythe optical gain adjusters 17a, 17b, 17c. On the other hand, since themonitor light is entered in the direction opposite to input lightdirecting to the optical gain adjusters 17a, 17b, 17c, the monitor lightdoes not adversely affect the input light in spite of the fact that themonitor light passes through the same optical fibers as the input lightdoes. Also, in this configuration, the optical coupler 33 is arrangedafter the optical isolator 13 to prevent input light split by theoptical power adjuster 8 from flowing through the optical coupler 33back into the erbium-added optical fiber 10.

As described above, the output monitor unit 34 can be realized by asimple configuration as illustrated in FIG. 11. In addition, thisconfiguration can be extended by increasing the number of branches ofthe 1×3 optical star coupler 37 even if the number of wavelengths oflight multiplexed on signal light is increased as λ₁, λ₂, λ₃,λ₄, . . . .

Next, FIG. 12 illustrates in detail the configuration of the controlunit 14 in the second embodiment. The control unit 14 comprises opticaldetectors 40a, 40b, 40c; a comparator circuit 41; a circuit 42 forproviding a predetermined reference value; a maximum error determinationcircuit 43; a selector circuit 44; an excitation light source 11; andoptical gain adjusters 17a, 17b, 17c.

In the illustrated configuration, the comparator circuit 41 comparesoptical power at respective wavelengths detected by the opticaldetectors 40a, 40b, 40c with a reference value provided by the circuit42 to output errors therebetween. The maximum error determinationcircuit 43 determines the wavelength at which the largest error isdetected. Then, the selector circuit 44 controls to transmit the largesterror at the wavelength to the excitation light source 11 as well as totransmit errors at the remaining wavelengths to the optical gainadjusters corresponding to the respective wavelengths (17b, 17c in FIG.12). In this way, the control unit 14 can control the excitation lightsource 11 such that optical power at the wavelength at which the largesterror has been detected among the detected optical power at therespective wavelengths reaches a predetermined value, and can alsocontrol simultaneously the optical gain adjusters corresponding to therespective wavelengths (17b, 17c in FIG. 2) such that optical power ateach of the remaining wavelengths reaches the predetermined value. Also,an optical gain adjuster which is not supplied with an error (17a inFIG. 12) provides a corresponding wavelength with a gain so as to alwaysminimize an optical loss. Here, the circuit 42 previously sets thereference value in such a manner that an error indicates a shortage ofexcitation power.

According to the configuration described above, the power of theexcitation light source 11 in the optical amplifier unit 9 is increased,only when optical power at a certain wavelength does not reach thepredetermined value even if an optical loss caused by the optical gainadjuster 17 is minimized. It is therefore possible to prevent anexcessive input of the excitation power. In addition, by simultaneouslycontrolling optical gain adjusters for wavelengths at which the largesterror is not present (17b, 17c in FIG. 12), the excitation power can bemade sufficient for any wavelength. This results in reducing powerconsumption and improving the reliability in the entire opticalamplifier.

In the alternative, the light splitter 33 and the output monitor unit 34illustrated in FIG. 11 may be modified as shown in FIG. 13.

Specifically, in FIG. 13, the output monitor unit 34 is composed ofoptical couplers 39a, 39b, 39c; light receivers 40a, 40b, 40c; anoptical coupler 45; and light combiner/separator units 15, 16. The lightcombiner/separator units 15, 16 have the same configurations as thoseillustrated in FIG. 5. However, the light combiner/separator units 15,16 in the second embodiment do not share the optical filters 20a, 20b,20c as in FIG. 5, and instead, a set of optical filters 20a, 20b, 20c isprovided for each of the light combiner/separator units 15, 16.

In the configuration illustrated in FIG. 13, part of an optical outputis split by an optical coupler 33 serving as a light splitter andinputted through the optical coupler 45 before the erbium-added opticalfiber 10 in the direction opposite to the flow of input light. However,in this configuration, the light combiner/separator unit 16 in theoptical power adjusting unit 8 is designed to combine light atrespective wavelengths adjusted by the optical gain adjusters 17a, 17b,17c as well as to separate light inputted in the direction opposite tothe flow of signal light into light at the respective wavelengths λ₁,λ₂, λ₃.

Light is separated by the light combiner/separator unit 16 into lightcomponents at the respective wavelengths which are then split by theoptical couplers 39a, 39b, 39c, respectively, to detect the powerthereof by the associated light receivers 40a, 40b, 40c.

In the configuration described above, the light combiner/separator unit16 is shared by the optical power adjusting unit 8 and the outputmonitor unit 34, so that the output monitor unit 34 can be implementedby a less number of components. Since the monitor light is inputted inthe direction opposite to the flow of signal light, the monitor lightdoes not adversely affect the input light in spite of the fact that themonitor light passes through the same optical fibers as the input lightdoes. Also, the optical coupler 33 is arranged after the opticalisolator 13 to prevent input light split by the optical power adjuster 8from flowing through the optical coupler 33 back into the erbium-addedoptical fiber 10. Further, a similar configuration may be used to copewith an increased number of multiplexed wavelengths such as λ₁, λ₂, λ₃,λ₄, . . . .

Alternatively, if a light path corresponding to an arbitrary wavelengthis not provided with the optical gain adjuster 17 as illustrated in FIG.4 (the light path for wavelength is not provided with the optical gainadjuster), the optical amplification apparatus of FIG. 13 may beconfigured as illustrated in FIG. 14. The configuration illustrated inFIG. 14 differs from the configuration illustrated in FIG. 13 in thatone optical gain adjuster 17 is omitted from the optical power adjustingunit 8. In this configuration, the control unit 14 controls theexcitation light source 11 in the optical amplifier unit 9 by detectingthe power of monitor light which is split from light at a wavelengthpassing through the light path on which no optical gain adjuster 17 isprovided. Light at the remaining wavelengths may be controlled byadjusting the associated optical gain adjusters 17b, 17c.

The foregoing configuration allows a reduction in the number ofcomponents required for the optical amplification apparatus. Inaddition, the control performed by the control unit 14 can be madesimpler.

The configuration of the control unit 14 for use in the opticalamplification apparatus configured as shown in FIG. 14 is illustrated inFIG. 15. The control unit 14 in FIG. 15 comprises optical detectors 40a,40b, 40c; a comparator circuit 41; a circuit 42 for providing apredetermined reference value; an excitation light source 11; andoptical gain adjusters 17b, 17c. The reference value provided by thecircuit 42 is determined such that a control amount outputted from thecomparator circuit 41 for the excitation light source 11 in the opticalamplifier unit 9 forces the optical amplifier unit 9 to adjust the powerof light at the wavelength passing through the light path, on which nooptical gain adjuster 17 is provided, to a predetermined level. Also,the reference value provided by the circuit 42 is determined such thatthe comparator circuit 42 supplies the optical gain adjusters 17b, 17cwith a control amount for adjusting the power of light at the remainingwavelengths to a predetermined level. In other words, the power of thelight at the wavelength passing through the light path on which nooptical gain adjuster 17 is provided is adjusted by increasing ordecreasing the power of the excitation light source 11, while the powerof the light at the wavelengths passing through the light paths on whichthe optical gain adjuster 17b, 17c are respectively provided is adjustedby increasing or decreasing an adjustment amount of the optical gainadjusters 17b, 17c, respectively.

Next, a third embodiment of the present invention will be described withreference to FIG. 16. The third embodiment is related to an opticalamplification apparatus applicable to the optical relay 4 or the opticalpre-amplifier 5 illustrated in FIG. 2.

FIG. 16 illustrates the configuration of the optical amplificationapparatus according to the third embodiment. As illustrated, the opticalamplification apparatus according to the third embodiment differs fromthe optical amplification apparatus according to the second embodimentin that an optical pre-amplifier 46 is additionally provided before theoptical power adjusting unit 8. The optical amplification apparatus ofthe third embodiment is intended to prevent a deteriorated S/N(signal-to-noise) ratio of the entire optical amplifier as well as toprevent a deteriorated S/N ratio of the entire optical transmissionsystem.

FIG. 17 illustrates in greater detail the optical amplificationapparatus according to the third embodiment. In FIG. 17, theconfigurations of the output monitor unit 34 and the optical poweradjusting unit 8 are identical to those in the optical amplificationapparatus illustrated in FIG. 14.

The optical pre-amplifier 46 is composed of an erbium-added opticalfiber 47 and a light combiner 48. An optical coupler 49 performs afunction of splitting excitation light from the excitation light source11 and inputting the split light to the erbium-added optical fiber 11 inthe optical amplifier unit 9 to excite the erbium-added optical fiber 11as well as a function of inputting the split light to an erbium-addedoptical fiber 47 in the optical pre-amplifier 46 to excite theerbium-added optical fiber 47.

In this configuration, the split ratio of the optical coupler 49 is setto 20:80, where the optical pre-amplifier 46 is coupled to a 20% branchwhile the optical amplifier unit 9 to an 80% branch. For example, iflight at wavelength λ₁ suffers a loss of -5 dBm in the optical poweradjusting unit 8, amplification of approximately 18 dB in the opticalpre-amplifier 46 will reduce deterioration of the S/N ratio in theentire optical amplification apparatus by approximately 60%. Inaddition, since the optical pre-amplifier 46 in this configurationsimultaneously amplifies light at wavelengths λ₁, λ₂, λ₃, deteriorationof the S/N ratio in the light at wavelength λ₁ and deterioration of theS/N ratio in the light at wavelength λ₂ are simultaneously reduced byapproximately 62% and approximately 65%, respectively.

Generally, in optical amplifiers, optical noise components called"naturally emitted light" are generated at wavelengths other than thoseof signal light, simultaneously with amplification of the signal light.The naturally emitted light may cause deterioration of the S/N ratio ofthe entire optical amplifier. However, in the third embodiment, theoptical filters 20 extract only light near wavelengths of associatedsignal light, the naturally emitted light components enteringsimultaneously with the signal light at wavelengths λ₁, λ₂, λ₃ from astage before the optical power adjusting unit 8 are removed by theoptical filters 20. Therefore, according to the configurationillustrated in FIG. 17, the deterioration of the S/N ratio as the entireoptical amplification apparatus is prevented from a viewpoint ofremoving the optical noise components.

Next, a fourth embodiment of the present invention will be describedwith reference to FIG. 18. The fourth embodiment is related to anoptical amplification apparatus applicable to the optical relay 4 or theoptical pre-amplifier 5 illustrated in FIG. 2.

FIG. 18 illustrates the configuration of the optical amplificationapparatus according to the fourth embodiment. As illustrated, the fourthembodiment differs from the first, second and third embodiments in thatthe positions of the optical power adjusting unit 8 and the opticalamplifier unit 9 are replaced with each other. Generally, since theoptical pre-amplifier 5 does not require an excessively large opticaloutput, output power and a deviation of power between wavelengths may beadjusted by the optical power adjusting unit 8 after the opticalamplifier unit 9. This configuration can prevent optical losses fromoccurring before the optical amplifier unit 9, so that an optical relay4 or an optical pre-amplifier 5 capable of reducing deterioration of theS/N ratio as the entire optical amplification apparatus can be providedin a simple configuration. It should be noted that in FIG. 18, thepositions of the power adjusting unit 8 and the optical isolator 13 maybe replaced with each other.

The first--fourth embodiments of the present invention have beendescribed. In each of the foregoing embodiments, the optical filter 20is arranged before each optical gain adjuster 17 in the optical poweradjusting unit 8. Alternatively, in the optical amplificationapparatuses other than that illustrated in FIG. 11, the optical filter20 may be arranged after each optical gain adjuster 17 instead ofarranging the optical filter 20 before each optical gain adjuster 17.

Specifically, the optical power adjusting unit 8 illustrated in FIG. 4may be modified such that the optical filter 20b is arranged between thelight combiner 23b and the optical star coupler 19, and the opticalfilter 20c is arranged between the light combiner 23c and the opticalstar coupler 19. Similarly, the optical amplification apparatus 8illustrated in FIG. 8 may be modified such that the optical filter 20ais arranged between the light combiner 23a and the optical star coupler19, the optical filter 20b is arranged between the light combiner 23band the optical star coupler 19, and the optical filter 20c is arrangedbetween the light combiner 23c and the optical star coupler 19. In theoptical amplification apparatuses illustrated in FIGS. 13, 14, 17, thelight combiner/separator unit 15 may be formed only of the optical starcoupler 18 without arranging the optical filters 20 therein.

Also with the modified configurations, light at each wavelength only isextracted from light amplified by each optical gain adjuster 17 by theoptical filter 20 arranged after each optical gain adjuster 17, in thesame manner as the configurations in which the optical filter 20 ispositioned before each optical gain adjuster 17, and the extracted lightis inputted to the optical star coupler 19, so that the light at eachwavelength adjusted by each optical gain adjuster 17 is combined by theoptical star coupler 19, similarly to the foregoing respectiveembodiments.

It will be now appreciated from the foregoing description that thepresent invention provides optical amplification apparatuses capable ofarbitrarily adjusting optical output power of light at respectivewavelengths and a deviation of power between the lights at therespective wavelengths.

What is claimed is:
 1. An optical booster amplifier for multiplexing aplurality of light signals each having a peculiar wavelength and foramplifying the multiplexed light signals, comprising:a plurality ofinput fibers; at least one optical gain adjusting unit receiving atleast one light signal from an input fiber selected from said pluralityof input fibers, adjusting an optical power of said at least one lightsignal, and outputting at least one adjusted light signal, wherein saidat least one gain adjusting unit is adjustable for both attenuating andamplifying; an optical coupler multiplexing said at least one adjustedlight signal together with other light signals from said plurality ofinput fibers to form a multiplexed light signal; and an opticalamplifier amplifying said multiplexed light signal.
 2. An opticalpre-amplifier for amplifying a wavelength multiplexed light signal andfor demultiplexing said wavelength multiplexed light signal to aplurality of wavelengths, comprising:an optical amplifier amplifyingsaid wavelength multiplexed light signal; an optical coupler receivingan amplified said wavelength multiplexed light signal, and dividing theamplified said wavelength multiplexed light signal to a plurality ofmultiplexed light signals; a plurality of optical band-pass filters eachreceiving a divided multiplexed light signal from said optical coupler,and each passing a peculiar band pass wavelength to output ademultiplexed light signal; and at least one optical gain adjusting unitreceiving said demultiplexed light signal from one of the optical bandpass filters, adjusting an optical power of said demultiplexed lightsignal, and outputting an adjusted light signal, wherein said at leastone gain adjusting unit is adjustable for both attenuating andamplifying.
 3. An optical pre-amplifier for amplifying a wavelengthmultiplexed light signal and for demultiplexing said wavelengthmultiplexed light signal to a plurality of light signals, comprising:anoptical amplifier amplifying said wavelength multiplexed light signal;an optical wavelength demultiplexer receiving and demultiplexing theamplified said wavelength multiplexed light signal to a plurality ofdemultiplexed light signals each having a perpendicular wavelength; andat least one optical gain adjusting unit receiving a demultiplexed lightsignal from the optical wavelength demultiplexer, adjusting an opticalpower of said demultiplexed light signal, and outputting an adjustedlight signal, wherein said at least one gain adjusting unit isadjustable for both attenuating and amplifying.
 4. An optical repeatingapparatus for repeating a wavelength multiplexed light signal whileapplying selective gain adjustment thereto, said apparatus comprising:awavelength demultiplexer receiving and demultiplexing an inputwavelength multiplexed light signal to a plurality of demultiplexedlight signals each having a peculiar wavelength; at least one opticalgain adjusting unit receiving a demultiplexed light signal from saidwavelength demultiplexer, adjusting an optical power of saiddemultiplexed light signal, and outputting an adjusted light signal,wherein said at least one gain adjusting unit is adjustable for bothattenuating and amplifying; an optical multiplexer receiving andmultiplexing both said adjusted light signal and said demultiplexedlight signals which do not pass through said optical gain adjustingunit, and outputting a repeated wavelength multiplexed light signal; anoptical amplifier amplifying said repeated wavelength multiplexed lightsignal; and a controller for controlling gain adjustment provided bysaid optical gain adjusting unit.
 5. An optical repeating apparatus forrepeating a wavelength multiplexed light signal while applying selectivegain adjustment thereto, said apparatus comprising:an optical amplifierfor amplifying an input wavelength multiplexed light signal; awavelength demultiplexer receiving and demultiplexing the amplified saidinput wavelength multiplexed light signal to a plurality ofdemultiplexed light signals each having a each peculiar wavelength; atleast one optical gain adjusting unit receiving a demultiplexed lightsignal from said wavelength demultiplexer, adjusting an optical power ofsaid demultiplexed light signal, and outputting an adjusted lightsignal, wherein said at least one gain adjusting unit is adjustable forboth attenuating and amplifying; an optical multiplexer receiving andmultiplexing both said adjusted light signal and said demultiplexedlight signals which do not pass through said optical gain adjustingunit, and outputting a repeated wavelength multiplexed light signal; anda controller for controlling gain adjustment provided by said opticalgain adjusting unit.
 6. An optical repeating apparatus according toclaim 4, further comprising:a coupler extracting at least one of aportion of said input wavelength multiplexed light signal and a portionof said repeated wavelength multiplexed light signal; a power monitorreceiving the at least one of the portion of said input wavelengthmultiplexed light signal and the portion of said repeated wavelengthmultiplexed light signal, and monitoring a power level of the same; andsaid controller receiving said power level from said power monitor andcontrolling gain adjustment in relation to said power level.
 7. Anoptical repeating apparatus according to claim 5, further comprising:acoupler extracting at least one of a portion of said input wavelengthmultiplexed light signal and a portion of said repeated wavelengthmultiplexed light signal; a power monitor receiving the at least one ofthe portion of said input wavelength multiplexed light signal and theportion of said repeated wavelength multiplexed light signal, andmonitoring a power level of the same; and said controller receiving saidpower level from said power monitor and controlling gain adjustment inrelation to said power level.
 8. An optical booster amplifier formultiplexing a plurality of light signals each having a peculiarwavelength and for amplifying the multiplexed light signals,comprising:a plurality of input fibers; at least one optical gainadjusting unit receiving at least one light signal from an input fiberselected from said plurality of input fibers, adjusting an optical powerof said at least one light signal, and outputting at least one adjustedlight signal, wherein said at least one gain adjusting unit isadjustable for both attenuating and amplifying; and an optical couplermultiplexing said at least one adjusted light signal together with otherlight signals from said plurality of input fibers to form a multiplexedlight signal.
 9. An optical pre-amplifier for amplifying a wavelengthmultiplexed light signal and for demultiplexing said wavelengthmultiplexed light signal to a plurality of wavelengths, comprising:anoptical coupler receiving said wavelength multiplexed light signal, anddividing said wavelength multiplexed light signal to a plurality ofmultiplexed light signals; a plurality of optical band-pass filters eachreceiving a divided multiplexed light signal from said optical coupler,and each passing a peculiar band pass wavelength to output ademultiplexed light signal; and at least one optical gain adjusting unitreceiving said demultiplexed light signal from one of the optical bandpass filters, adjusting an optical power of said demultiplexed lightsignal, and outputting an adjusted light signal, wherein said at leastone gain adjusting unit is adjustable for both attenuating andamplifying.
 10. An optical pre-amplifier for amplifying a wavelengthmultiplexed light signal and for demultiplexing said wavelengthmultiplexed light signal to a plurality of light signals, comprising:anoptical wavelength demultiplexer receiving and demultiplexing saidwavelength multiplexed light signal to a plurality of demultiplexedlight signals each having a perpendicular wavelength; and at least oneoptical gain adjusting unit receiving a demultiplexed light signal fromthe optical wavelength demultiplexer, adjusting an optical power of saiddemultiplexed light signal, and outputting an adjusted light signal,wherein said at least one gain adjusting unit is adjustable for bothattenuating and amplifying.
 11. An optical repeating apparatus forrepeating a wavelength multiplexed light signal while applying selectivegain adjustment thereto, said apparatus comprising:a wavelengthdemultiplexer receiving and demultiplexing an input wavelengthmultiplexed light signal to a plurality of demultiplexed light signalseach having a peculiar wavelength; at least one optical gain adjustingunit receiving a demultiplexed light signal from said wavelengthdemultiplexer, adjusting an optical power of said demultiplexed lightsignal, and outputting an adjusted light signal, wherein said at leastone gain adjusting unit is adjustable for both attenuating andamplifying; an optical multiplexer receiving and multiplexing both saidadjusted light signal and said demultiplexed light signals which do notpass through said optical gain adjusting unit, and outputting a repeatedwavelength multiplexed light signal; and a controller for controllinggain adjustment provided by said optical gain adjusting unit.
 12. Anoptical repeating apparatus for repeating a wavelength multiplexed lightsignal while applying selective gain adjustment thereto, said apparatuscomprising:a wavelength demultiplexer receiving and demultiplexing saidinput wavelength multiplexed light signal to a plurality ofdemultiplexed light signals each having a each peculiar wavelength; atleast one optical gain adjusting unit receiving a demultiplexed lightsignal from said wavelength demultiplexer, adjusting an optical power ofsaid demultiplexed light signal, and outputting an adjusted lightsignal, wherein said at least one gain adjusting unit is adjustable forboth attenuating and amplifying; an optical multiplexer receiving andmultiplexing both said adjusted light signal and said demultiplexedlight signals which do not pass through said optical gain adjustingunit, and outputting a repeated wavelength multiplexed light signal; anda controller for controlling gain adjustment provided by said opticalgain adjusting unit.